1. Field of the Invention
This invention relates to a radiation image processing method and apparatus, in which a blackening process is carried out on a radiation image signal representing a radiation image, such that a region outside of an irradiation field on the radiation image may become darker than the region inside of the irradiation field. This invention particularly relates to an improvement in processing carried out in cases where the shape and location of the irradiation field were determined incorrectly.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image having good image quality by use of the processed image signal have heretofore been known in various fields. For example, as such techniques, the applicant proposed various radiation image recording and reproducing systems which use stimulable phosphor sheets.
When a radiation image of an object, such as a living body, is recorded on a recording medium, such as X-ray film or a stimulable phosphor sheet, it is desirable that adverse effects of radiation upon the living body can be kept as small as possible. Also, if object portions not related to a diagnosis, or the like, are exposed to radiation, the radiation will be scattered by such portions to the portion that is related to a diagnosis, or the like, and the image quality will be adversely affected by the scattered radiation. Therefore, when a radiation image is recorded on the recording medium, an irradiation field stop is often used in order to limit the irradiation field to an area smaller than the overall recording region of the recording medium, such that radiation may be irradiated only to that portion of the object, the image of which is to be used.
In cases where a radiation image is recorded on a recording medium, such as a stimulable phosphor sheet, by using an irradiation field stop, an image of an object, or the like, is recorded in a region (i.e., an irradiation field) inward from the aperture contour of the irradiation field stop. Also, a region outward from the aperture contour of the irradiation field stop is not exposed to the radiation. Therefore, the aperture contour of the irradiation field stop constitutes an edge of the image.
In cases where an image signal is detected from the recording medium, on which an image has been recorded within only the irradiation field, and image processing is carried out on the image signal, the image processing, such as gradation processing, may be carried out on only the image signal components of the image signal, which correspond to the region inside of the irradiation field. In this manner, the amount of the processing can be reduced markedly, the load of the processing can be kept small, and the processing speed can be kept high.
The shape and location of the irradiation field can be determined automatically by, for example, utilizing the characteristics in that the aperture contour of the irradiation field stop constitutes the edge of the image and finding a portion, at which the value of the image signal changes sharply. Various techniques for determining the shape and location of the irradiation field have been proposed in, for example, U.S. Pat. No. 4,967,079.
An image signal, which has been obtained from the image processing, is fed into, for example, a display means, such as a cathode ray tube (CRT) display device, or a reproducing means, such as a laser printer, and used for reproducing a visible image.
However, in cases where the reproduced visible image is displayed on a CRT display device, the luminance of the region outside of the irradiation field becomes approximately highest on the visible image. Also, in cases where the visible image is reproduced on photographic film, the image density of the region outside of the irradiation field becomes approximately lowest on the visible image. Therefore, when the reproduced visible image is seen, strong light comes from the region outside of the irradiation field on the visible image and enters the eyes of the person, who sees the visible image. Accordingly, even if the image within the irradiation field has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness, the reproduced visible image gives a glaring feeling to the person, who sees it, and becomes hard to see.
In order for the aforesaid problems to be eliminated, a process for blackening the region outside of the irradiation field has heretofore been carried out. With the blackening process, the image signal values corresponding to the region, which is outside of the irradiation field and to which no radiation impinged in the radiation image recording operation, and representing approximately the highest luminance or approximately the lowest image density are forcibly replaced by image signal values representing a low level of luminance or a high level of image density. In this manner, with the blackening process, the region outside of the irradiation field is rendered dark, such that the image within the irradiation field can be seen easily. The blackening process is proposed in, for example, Japanese Unexamined Patent Publication No. 3(1991)-98174.
However, the aforesaid processing for automatically determining the shape and location of the irradiation field cannot necessarily be carried out accurately. It often occurs that an area, which is larger or smaller than the area of the correct irradiation field, is determined as the irradiation field by mistake. If an area, which is larger than the area of the correct irradiation field, is determined as the irradiation field, a gap will occur between the correct irradiation field and the determined irradiation field, and the blackening process will not be carried out with respect to the gap. In such cases, substantially the same problems will occur as those occurring when the blackening process is not employed. If an area, which is smaller than the area of the correct irradiation field, is determined as the irradiation field, a portion of the correct irradiation field will be subjected to the blackening process. It will often occur that a portion of the object image, which is necessary for a diagnosis, or the like, is located at the thus blackened portion of the irradiation field. In such cases, it becomes necessary to again carry out the operation for recording the radiation image.